Allowed Values

Allowed values are the closed set of values a given event property is permitted to take; anything outside that set is treated as an error or as data pollution.

Its value is in preventing sprawl: without an allowed set, a status property drifts into paid, Paid, PAID and pd, splitting one fact into several values no report gathers together. Allowed values are really the part of an event schema that focuses on the value rather than the property name. The main trap is over-tightening: too closed a set rejects a new, legitimate value — a fresh payment method, say — and real data is lost, so an allowed-value set must be defined alongside a clear path for adding a new value, or it turns from protection into obstruction.